1. Field of the Invention
The present invention pertains to a process for the convenient, high-yield preparation of base-free carbazolide anions.
2. Background Art
Carbazolide anions have numerous uses in synthetic organic and organometallic chemistry. As but one example, transition metal complexes of carbazolides are known to have catalytic activity, i.e. as polymerization catalysts for xcex1-olefin polymerization as disclosed in U.S. Pat. 5,539,124. In the syntheses of these compounds, the metal chloride is allowed to react with two equivalents of carbazolide anion. However, syntheses of many carbazolide complexes and carbazole derivatives require the use of substantially base-free carbazolide anions, i.e. potassium carbazolide. The preparation of base-free compounds has proven difficult.
For example, the deprotonation of carbazole by butyllithium in a toluene or pentane solvent is slow and incomplete, perhaps due to the possibility of hydrogen bonding in the partially deprotonated reaction mixture. Appler (J. Organometal. Chem., 350 (1988) 217) reported that deprotonation of carbazole by butyllithium in toluene required refluxing at 110xc2x0 C. With the addition of coordinating solvents such as tetramethylethylene diamine (TMEDA) in tetrahydrofuran (THF) or toluene solvent, deprotonation is more rapid and more complete. However, the product is a base-coordinated complex such as [Li(THF)2][C12H8N] (Hacker et al., Chem. Ber., 120 (1987) 1533) or [Li(TMEDA)][C12H8N]. In subsequent reaction with Group 4 metal compounds such as TiCl4, Zr4Cl4, HfCl4, the Lewis base remains coordinated in the final product to form (C12H8N)2MCl 2(L)2 compounds where L is THF or xc2xd TMEDA. These products are difficult to characterize and subsequent chemistry is complicated by the presence of the solventing group.
It would be desirable to provide a synthesis of carbazolide salts which are free of Lewis bases, producing the carbazolide salt in reasonable yields and with reasonable reaction times.
It has now been surprisingly discovered that carbazolide ions free of Lewis bases and complexing neutral ligands can be prepared rapidly and in high yield by reacting carbazole with a deprotonating agent in hydrocarbon solvent in the presence of at least an equimolar amount of an aprotic coordinating ligand to form an intermediate complex, followed by reaction in hydrocarbon solvent in the presence of one equivalent of alkali alkoxide to yield the base-free alkali carbazolide.
Any carbazole is believed to be useful in the present process. Preferred carbazoles correspond to the formula 
where R1 through R8 are hydrogen or substituents which do not interfere with obtaining the desired carbazolide anion product. Preferred R1 through R8 are hydrogen, alkyl, alkenyl, cycloalkyl, cycloalkenyl, alkoxy, polyoxyalkyl, aryl, heteroaryl, silyl, or two adjacent R1 through R8 may form a C4-6 cycloalkyl or 6-membered aryl or heteroaryl ring or a C4-6 cycloalkyl or 6 membered aryl or heteroaryl portion of a more extended ring system. Any of R1 through R8 and more preferably any of R2 through R7 and yet more preferably any of R3 through R6 may be an alkylene, alkenylene or silyl, etc. bridging linkage between a first carbazolide anion and a second carbazolide anion. Any one of Cxe2x80x94R1 to Cxe2x80x94R8 can also be replaced by a nitrogen, phosphorous, or arsenic atom.
Preferably, R1 through R8 are individually hydrogen, C1-4 lower alkyl, or one or more pairs of adjacent R1 through R8 (including the pair of R4 and R5) may form an aryl or heteroaryl ring or an aryl or a heteroaryl portion of a larger aromatic ring system. Further examples of preferred carbazoles are found in U.S. 5,539,124, incorporated herein by reference. When R1 through R8 are alkyl, they are preferably C1-4alkyl, more preferably methyl. It is preferred that R1 and R8 are hydrogen.
The hydrocarbon solvent in which the reaction takes place is a non-complexing hydrocarbon solvent which may also contain heteroatoms such as O, S, or N, as long as the heteroatoms do not facilitate strong complexing of the solvent with the carbazolide product. Most preferably, the solvent is an aromatic, aliphatic or cycloaliphatic hydrocarbon solvent. Preferred cycloaliphatic solvents include cyclopentane, cyclohexane, and cycloheptane. Preferred aliphatic solvents include pentane, hexane, heptane and their branched isomers. Preferred aromatic solvents include benzene, toluene, or o-, m-, or p- xylenes. Mixtures of solvents may be used. The preferred solvent is toluene. Coordinating solvents such as diethyl ether, tetrahydrofuran, and similar ether solvents can be used, but the intermediate lithium salt must be isolated from the reaction mixture for use in the second step of the reaction.
The coordinating ligand is a Lewis base-containing ligand, preferably an aprotic nitrogen-or oxygen-containing ligand. Preferred coordinating ligands include monobasic ligands such as tetrahydrofuran, 1,4-dioxane, and pyridine; chelating dibasic ligands such as N,N,N,N-tetramethylethylenediamine and ethylene glycol dimethyl ether; chelating tribasic ligands such as 1,3,5-trioxane and 1,3,5-trimethyl-1,3,5-triazine; and chelating tetrabasic ligands such as 12-crown-4. Dibasic and polybasic ligands containing both nitrogen and oxygen functionalities as well as ligands containing non-nitrogen and -oxygen complexing Lewis base atoms such as P, O, or S, may also serve as coordinating ligands.
The alkali alkoxide is one which is capable of destroying the intermediate lithium carbazolide/dibasic ligand complex. In general, the alkali alkoxide may be selected from sodium, potassium, and cesium alkoxides, but is preferably a potassium alkoxide. The alkoxide ion may have 1 to 10 or more carbon atoms, thus including methoxide, ethoxide, n-propoxide, isopropoxide, n-butoxide, i-butoxide, s-butoxide, t-butoxide, and other ions derived from linear, branched, or cyclic alkanols. The length of the carbon chain should be such that the resultant lithium alkoxide is soluble in aliphatic, cycloaliphatic, or aromatic solvents. Preferably, the alkali alkoxide is potassium t-butoxide.
The reaction may take place in one or more stages and will be described with reference to the preferred preparation using carbazole and TMEDA as the coordinating ligand. First, the carbazole is reacted in hydrocarbon solvent with alkyl lithium, preferably n-butyl lithium, in the presence of an equimolar amount of TMEDA, at a convenient temperature, i.e. from xe2x88x9220xc2x0 to 50xc2x0 C., preferably, 0xc2x0 C. to 25xc2x0 C. The reaction takes place preferably under moisture-free conditions, preferably under an inert gas blanket, i.e. of nitrogen, argon, etc. Solvents should be dried by conventional techniques. Following the reaction, the complex [Li(TMEDA)][C12H8N] may be collected as a white crystalline solid, substantially free of starting material. The intermediate may be separated and washed with additional solvent, or may be retained in the reaction solvent without isolation. If isolated, the intermediate is slurried in toluene or other solvent and reacted with one equivalent of potassium t-butoxide. This reaction may be carried out at temperatures from 0xc2x0 C. to reflux, preferably room temperature to reflux. Potassium carbazolide is isolated as a white crystalline solid and optionally washed with additional solvent to remove all traces of lithium alkoxide. Lithium t-butoxide washes away in the solvent, having substantially pure potassium carbazolide. 1H NMR generally shows no trace of starting material.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.